Waviness alters both geometric and mechanical properties of stochastic fibrous networks and significantly affects overall mechanical response, but few results are available in the literature on the subject. In this work, we explore the importance of the dimension of constituent fibers (1D vs 2D) in determination of percolation thresholds, and other fundamental statistical properties of fibers having geometric waviness, in adaptation of classical theories on random lattices to practical applications, including analysis of nanotube ropes and collagen bundles. Although the so-called “curl ratio” clearly affects the statistical properties, as evaluated by Kallmes and Corte a few decades ago, we have found some results in this classic work to be inaccurate for systems containing fibers of moderate waviness. Our main findings include the independence of the mean number of crossings with regard to waviness, as well as the nonlinear dependence of probability of intersection on waviness. Our investigation of percolation in wavy fiber networks reveals that the percolation threshold is significantly increased, with increasing curl ratio. In addition, several nontrivial results related to network properties of infinite straight lines are also described, some of which are believed to have wide applications in practice.

Accumulation and annealing of damage in implanted with self-ions to high doses were investigated using a combination of grazing incidence diffuse x-ray scattering, high-resolution x-ray diffraction scans, and transmission electron microscopy. During implantation at , small vacancy and interstitialclusters formed at low doses, but their concentrations saturated after a dose of . The concentration of Frenkel defects at this stage of the implantation was . At doses above , the concentration of implantedinterstitial atoms began to exceed the Frenkel pair concentration, causing the interstitialclusters to grow, and by , these clusters formed dislocation loops. Kinematical analysis of the rocking curves illustrated that at doses above the “plus one” model was well obeyed, with one interstitial atom being added to the dislocation loops for every implanted atom. Measurements of Huang scattering during isochronal annealing showed that annealing was substantial below for the specimens irradiated to lower doses, but that little annealing occurred in the other samples owing to the large imbalance between interstitial and vacancy defects. Between 700 and a large increase in the size of the interstitialclusters was observed, particularly in the low-dose samples. Above , the interstitialclustersannealed.

An atomic-scale graded structure has been formed previously in a (at. %) alloy that is miscible in all proportions, by sedimentation of substitutional solute atoms under an ultrastrong gravitational field up to at 220–240 °C [T. Mashimo, T. Ikeda, and I. Minato, J. Appl. Phys. 90, 741 (2001)]. In this study, additional megagravity field experiments were performed on the alloy and pure Bi at different temperatures below their melting points, to investigate the change in crystalline state under the ultrastrong gravitational field. For the alloy ultracentrifuged at 191–205 °C, no change in composition was observed, and the grain sizes of the crystals decreased from several millimeters to tens of micrometers, while no distinct change in grain size was observed for the pure Bi ultracentrifuged under the same experimental conditions. The alloy ultracentrifuged at 220–240 °C consisted of two regions with different morphologies–fine-grained crystals with grain sizes in the range of tens of micrometers in the low gravity region, and large crystals with grain sizes several millimeters long and hundreds of micrometers wide along the direction of gravity in the high gravity region, where sedimentation of atoms was confirmed. The large crystals with hexagonal structures were formed by preferential crystal growth roughly along the c axes, and a large strain that increased as the gravitational field increased existed inside these crystals. Formation of this anomalous crystal state might be correlated with the sedimentation of atoms.

The deep levels in GaN associated with yellow luminescence transitions have been investigated using photoluminescence, Hall measurements, and deep level transient spectroscopy(DLTS). Hall measurements on Si-doped GaN show the presence of donor levels at ∼18, ∼35, and ∼70 meV, which are respectively associated with the Si shallow donors, O impurities, and the nitrogen vacanciesDLTS measurements, on the other hand, reveal trap levels at and The trap level at obtained from DLTS can be correlated to the 70 meV deep donor obtained from Hall measurements. The deep donor band at is attributed to the related defect complex decorated along dislocation sites while the hole level at is attributed to the Ga vacancy Thermal annealing at 750 °C in nitrogen ambient results in reduction of yellow luminescence, which could be due to decrease in the concentration of and -related defect complexes. From these observations, we propose that yellow luminescence in GaN arises from the transitions from the levels to the deep level at

Intrinsic tensile stress, which can lead to problems in deposited thin films such as cracking, peeling, and delamination, often develops during the early stages of thin film growth. Many attempts have been made to estimate the tensile stress during crystallite coalescence, both experimentally and analytically. Most recently, using a combination of Hertzian contact mechanics and elasticity theory, Freund and Chason applied the Johnson-Kendall-Roberts (JKR) theory to account for adhesion between crystallites under specific conditions. Other existing contact mechanics models that naturally account for adhesion include the improved Derjaguin-Muller-Toporotov and Maugis-Dugdale theories. The objective of this study is to provide useful analytical and numerical techniques based on these contact mechanics theories for a wide range of conditions that accurately approximate the intrinsic tensile stress that develops during crystallite coalescence. As an analytical method, the Maugis-Dugdale model is proposed as a more general alternative to the JKR model. Parameters such as the contact radius and “net” adhesive force are computed as a function of the relative separation between two adjacent crystallites in a thin goldfilm. Another useful parameter known as the “jump-to-contact” separation is also calculated by the Maugis-Dugdale and JKR models. For comparison to the analytical models, a finite element method is used to simulate the crystallite coalescence problem. The numerical technique is based on a nonlinear surface interaction element developed to approximate van der Waals adhesion, and allows for full-field analysis of stress and displacement in crystallites. Two different boundary conditions are used, for which corresponding contact radius and tensile stress are computed and compared to the analytical results. As a further study, the length scale effect is also investigated by varying the radius of individual crystallites from to . It is concluded that in order to estimate the average tensile stress accurately using analytical models, the radius of individual crystallites must be large compared to the contact radius. For small length scales, the finite element approach is more appropriate.

ion-doped nanocrystals were prepared by coprecipitation synthesis technique. Visible emissions at , , and have been observed under excitation at . Comparing with the stokes emission characteristics of the nanocrystals pumped at , the up-conversion mechanisms excited at have been investigated in detail. Excitation power dependent behaviors of the up-converted emission intensities indicate that a two-photonexcited stateabsorption process is responsible for the transition while a no-resonant energy transfer principally performs the red up-conversion of the transition.

The effect of growth temperature and postmetallization annealing on the electrical characteristics of atomic layer depositedfilms has been studied. Trap distributions at the interface have been obtained by means of deep level transient spectroscopy, whereas conductance transient technique provided the energy and geometrical profiles of electrically active defects at the insulator bulk. Differences in the interface quality have been observed between films on - and -type substrates. The most suitable growth temperatures seem to be for -type and for -type substrates. In all cases, postmetallization annealing in forming gas causes displacements of defects from the insulator bulk to the interface.

High lateral density quantum dot structures are self-formed by growing short-period superlattices on substrates by gas sourcemolecular beam epitaxy.Multilayer structures sandwiched with barrier layers showed a strong photoluminescence emission. The wavelength emission was easily obtained and precisely controlled by regulating the period as well as the number of monolayers (layer thickness). This wavelength control can be understood by considering the carrier confinement along the growth direction as well as the effective well depth change.

Diffusion of in is studied in the temperature range using implantation doping and B dopedepitaxialGe layers. Concentration profiles before and after furnace annealing were obtained using high resolution secondary ion mass spectroscopy (SIMS). Diffusion coefficients were calculated by fitting the annealed profiles using TSUPREM. We obtained diffusivity values which are at least two orders of magnitude lower than the lowest values previously reported in the literature. Using our values an activation energy of is calculated. Present experimental results suggest that interstitial mediated mechanism should be considered for diffusion in in accordance with recent theoretical calculations. Annealed SIMS profiles also suggest that solid solubility in is at which agrees with literature values.

template layer strain effects on the growth of light emitting diodes devices were investigated. Seven-period multiple quantum well structures were deposited on and template layers. It was found that the electroluminescence emission of the device was redshifted by approximately . Triple-axis x-ray diffraction and cross-sectional transmission electron microscopy show that the template layer device was virtually unstrained while the layer experienced tensile strain. Dynamic secondary ion mass spectrometry depth profiles show that the template layer device had an average indium concentration of higher than that of the template layer device even though the structures were deposited during the same growth run. It was also found that the layer device had a higher average growth rate than the template layer device. This difference in indium concentration and growth rate was due to changes in thermodynamic limitations caused by strain differences in the template layers.

single crystals with , 0.30, 0.31, and 0.33 have been investigated by spectroscopic ellipsometry. The refractive indices and extinction coefficients were obtained. The modified Sellmeier equations for the refractive indices were obtained by least-squares fit. The equations can be used to calculate the refractive index with high accuracy in the low absorption wavelength range, namely, from 400 to . The Sellmeier optical coefficients , , , and were calculated by fitting the single-term oscillator equation. They are related directly to the electronic energy band structure and have the physical significance. The optical band gapenergies were also obtained from absorption coefficient spectra. Our results show that as the PT content increases, the refractive index of single crystals increases, while the optical band gapenergy decreases. Some discussions about the octahedron building block that determines the basic energy level of single crystals are also presented in this article.

Doping single-crystalline -quartz with -ion implantation under the conditions of dynamic solid phase epitaxial regrowth has been studied as function of ion fluence and substrate temperature. In particular, the light emitting properties possibly suitable for optoelectronic devices have been investigated by measuring cathodoluminescence spectra for implantation temperatures from 300 to and for analyzing temperatures from . Rutherford backscattering channeling analysis showed that the implantation produced amorphous layers varying in depth with temperature. At a fluence of and an implantation temperature of , implantation is accompanied by a strong increase in the luminescence intensity of a violet band, which we associate with -related defects or clusters. This violet band is very stable and has a long lifetime of . All the other bands observed are connected to known oxygen defect centers in the network.

This work presents a study of intrinsic zinc oxide thin film as ozone sensor based on the ultraviolet photoreduction and subsequent ozone re oxidation of zinc oxide as a fully reversible process performed at room temperature. The films analyzed were produced by spray pyrolysis, and magnetron sputtering. The resistivity of the films produced by magnetron sputtering and constituted by nanocrystallites changes more than eight orders of magnitude when exposed to an dose of . On the other hand, porous and textured zinc oxide films produced by spray pyrolysis at low substrate temperature exhibit an excellent ac impedance response where the reactance changes by more than seven orders of magnitude when exposed to the same dose, with a response frequency above , thus showing improved ozone sensing discrimination.

single crystals were grown by using the Bridgman method. Hall-effect measurements showed that the carrier type and carrier concentration of unintentionally crystals were p type and between approximately and , respectively. Phototransmission spectra showed that the position of the band edge emission shifted to higher wavelength with increasing mole fraction, and photoluminescence spectra showed that the peak corresponding to the excitons bound to neutral acceptors shifted to lower energy with increasing mole fraction. The electro-optic sensors fabricated utilizing the single crystals were operated in the terahertz spectrum range. These results can help improve the understanding of single crystals for applications in terahertz electro-optic sensors.

In the present study crystals with vicinal surfaces inclined and towards and , respectively, have been used to deposit iridium layers and subsequently epitaxialdiamondfilms. The iridiumgrows on top of the vicinal with a perfect cube-on-cube registry. In contrast, the diamondfilms on the iridium show a strong additional tilting of up to . A model is proposed which explains the effect by the preferential incorporation of misfit dislocations due to the stress anisotropy on the vicinal substrate planes taking into account the lattice misfit and the islandlike growth mode of diamond. The experimental observations and their explanation suggest that the epitaxialdiamond alignment on iridium takes place during lateral growth within the first few nanometers.

We report studies of the effect of rapid thermal annealing(RTA) on the optical properties of a low-loss saturable Bragg reflector (SBR), consisting of a single quantum well embedded in an Bragg reflector grown monolithically on a substrate. RTA gives rise to a blueshift of the photoluminescence(PL) peak (and therefore of the excitonic absorption peak) and an enhancement of PL intensity, while the reflectivity properties including peak reflectivity and bandwidth are not degraded. Temperature dependent photoluminescencemeasurements show that the RTA-induced blueshift of photoluminescence consists of two components: one originating from the increase of optical transition energies and another from the reduction of carrier localization. Time-resolved photoluminescence results at room temperature provide information about the recombination dynamics of carriers directly relevant to the application of the SBR in laser mode locking.

The nonlinear optical phase shift in self-assembledquantum dots under resonant excitation in a ground-state transition was measured by a unique two-color pump∕probe ellipsometric polarization analysis. This ellipsometric analysis makes use of the large optical birefringence of [(SK) — Stranski-Krastanov] originating from the asymmetric structure. A phase shift of rad was obtained at an input pump pulse energy density of , a detuning of , and a time delay of in a long waveguide having with a peak wavelength of , a volume density of , and inhomogeneous broadening of . Analysis revealed that the phase shift is mainly attributed to the absorption saturation for -polarized light, though other mechanisms also could contribute at higher pumping. The calculation, based on the two-level approximation, revealed that the minimum energy density for shift is , calculated under ideal conditions.

The effect of fluorine incorporation on properties of silicon dioxide thin films has been studied as a function of gas flow ratio. Fluorine was incorporated into silicon dioxide films during high density plasma chemical vapor deposition with the gas mixture used to improve the gap-filling ability for shallow trench isolation of devices. Refractive index measured by ellipsometry decreased with increasing flow ratios for both as-deposited and annealedfilms. X-ray reflectivity measurements showed that both fluorine incorporation and thermal annealing reduced the film density. The analysis of infrared absorption spectra showed the relaxation of the bond with increasing flow ratios and thermal annealing. The secondary ion mass spectroscopy and x-ray photoelectron spectroscopy studies confirmed the behavior of fluorine diffusion and the binding energy for each species in the films, respectively. These results showed that through fluorine incorporation and thermal annealing, the network structures of silicon dioxide could be modified from low order rings to high order rings accompanied by the enlargement of nanovoids.

We have investigated the photoluminescence spectra of silicon-based nanoscale islands obtained by anodization of silicon-based aluminum membranes in a sulfuric acid solution under a constant voltage of . Two ultraviolet emission bands were observed at 290 and . After annealing the samples in in , the band vanishes, but the band still exists. We suggest that the band originates from optical transition in the centers in the islands according to its annealing behavior. The band is considered to be from -related luminescence centers, , because a decrease of intensity of the band is in agreement with that of amount of the ion impurities located in the islands. This work shows a clear understanding of the light-emitting mechanism of silicon-based island array. The obtained result can be expected to have important applications in modem optoelectronics.

X-ray diffuse scattering is used to probe size, shape, and strain distribution of self-organized islands, which were grown by liquid phase epitaxy. The islands show a truncated pyramidal shape with {111} side facets and a (001) top facet and they are highly uniform in size. With an averaged island base width of and a corresponding height of all the islands have a characteristic geometrical base-to-height aspect ratio of about 2. X-ray diffuse scattering is used to locally probe the elastically relaxed regions inside the island apex and the strongly strained regions near the substrate-island interface. It is found that the geometrical aspect ratio has a large impact on the x-ray diffuse intensity pattern in reciprocal space. By performing corresponding kinematical x-ray simulations this fact can be utilized to determine the aspect ratio with high sensitivity.